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FAQ: Screening Quick Reference Tables (SQuiRTs)
Q. Will the cards be updated? If so, how often might that happen?
A. Yes, we plan to release updates whenever changes or additions are needed. A Nov 2006 update incorporates changes in Ambient Water Quality Criteria for cadmium, copper, and tin (pages 3 and 4), plus makes some minor additions.
Q. Why aren't full references provided?
A. Although we would have liked to have provided full references on the SQuiRT cards, space did not allow us to provide full bibliographic citations tied to each and every datum. Because some columns contain data from as many as six different sources, we could not include full references.
Q. On page 2, what does the "Increasing" predicted toxicity gradient mean?
A. Each sediment quality guideline is intended to describe a particular--but unique--point in a spectrum of toxicity. These points range from lower thresholds, below which samples are presumably non-toxic, to upper thresholds of toxicity, above which samples are predicted. The guidelines have been ordered according to the level of toxicity each is intended to describe, from low to high.
Q. Why don't the individual chemical values always increase across the predicted toxicity gradient?
A. The individual measurement endpoint for a UET or AET may be more sensitive than a PEL or ERM value, since PEL and ERM values incorporate several endpoints in their determination. A UET, therefore, may have a lower value than a PEL.
Q. Why are background levels included?
A. Background levels have no development basis in risk characterization or toxicity evaluation. However, for purposes of screening samples which may be indicative of contaminant source areas (the main purpose of the SQuiRT cards is to aid you in doing this screening), background concentrations are useful for distinguishing possible contaminant sources.
Q. Why does NOAA apply a default dilution factor of only 10x for the discharge of ground water to surface water?
A. We prefer to use site-specific information whenever it is available. But because such data have not been derived, we acknowledge that some level of dilution would occur. We chose to use a conservative, order of magnitude dilution factor for screening purposes to ensure a high degree of confidence that any contaminant source eliminated from further consideration is not likely to pose substantial risk. Conversely, this is not meant to imply that contaminant sources that do not pass this screening do pose risk.
Q. Why isn't the value for total DDTs in the marine TEL/PEL, ERL/ERM, or AET benchmarks simply the sum of the individual three isomers?
A. This is a great example of why an understanding of the derivation of these various benchmarks is key to their proper application. The total DDT benchmark is not simply a mathematical summation of the other three benchmarks. It is derived by the same data evaluation process as any other compound. Therefore, it is responsive to the most potent of the isomers that were present in the samples used to generate the benchmark.
The total DDT benchmark should only be used for screening when there is no more specific information about the constituent composition of “total DDT.” Because each isomer has its own potency, individual concentrations of each should be screened whenever possible.
Other pages in this series
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